ALBUQUERQUE, N.M.— Lesser prairie chicken population numbers dropped by more than 50 percent over the past year, according to a study released today. The finding raises questions about the adequacy of voluntary conservation measures proposed today for the rare grouse in a final rangewide conservation plan intended to preclude the need for Endangered Species Act protections.

“Drought and habitat destruction are devastating the small remaining populations of this magnificent grassland bird,” said Jay Lininger with the Center for Biological Diversity. “Voluntary measures are too little, too late, and will not get traction fast enough to prevent extinction. These vanishing birds need the protection that only the Endangered Species Act can provide if they’re going to survive.”

The study, by Western EcoSystems Technology Inc. of Laramie, Wyo., estimates the total population size at 17,616 individuals in 2013, more than a 50 percent drop from the 2012 estimate of 34,440 birds. The study also estimated there to be 2,036 occupied breeding areas (known as leks) in 2013 — a decline of more than 30 percent from the 2012 estimate of 2,930 leks.

Prairie chicken habitat has declined overall by as much as 92 percent, according to federal scientists, and threats from habitat loss and fragmentation will increase with proposed energy developments, agricultural conversions and other land uses anticipated under the new rangewide conservation plan. Only 71 patches of habitat as large as 25,000 acres — the area required for effective chicken strongholds — exist within the entire five-state occupied range.

The conservation plan announced today sets a 10-year population goal of 67,000 prairie chickens range-wide. However, it would designate “focal areas” of habitat that are less than half the size required to maintain breeding populations. The total acreage of the focal areas is less than 35 percent of the bird’s currently occupied habitat and only 6 percent of the historical range of the species.

“We’re disappointed the plan locks the lesser prairie chicken into small areas of habitat, precludes their recovery, and gives blanket approval to industrial activities that are pushing them to extinction,” Lininger said.

The prairie chicken was proposed for Endangered Species Act protection in December, but under a special rule added to the proposal by the U.S. Fish and Wildlife Service in May, habitat-destroying activities by parties that voluntarily adopt the new plan’s mitigation strategy would be allowed to continue.

Lesser prairie chickens have been on the waiting list for federal protection since 1998. The species was proposed for listing as a result of a 2011 settlement between conservation groups and the Fish and Wildlife Service to speed protections for hundreds of species around the country.

An indicator species for the southern Great Plains, these medium-sized, gray-brown grouse live in shinnery oak and sand-sagebrush grasslands in parts of Colorado, Kansas, New Mexico, Oklahoma and Texas. Their range has been reduced by more than 90 percent, and their population has declined by approximately 85 percent, since the 1800s. The Fish and Wildlife Service identified continued population declines and myriad land uses as threats to the species’ persistence.

The lesser prairie chicken is threatened by habitat loss and degradation from livestock grazing, agriculture, oil and gas extraction and herbicides. Habitat fragmentation from fences and power lines and disturbance from roads, mining and wind-energy production also affect the species. Climate change and drought are increasingly important threats.

Like other western grouse, male lesser prairie chickens engage in a unique, elaborate, and comical communal breeding display each spring to attract females. Both males and females congregate at breeding grounds (leks), where the males strut (“dance”), vocalize (“boom”) and physically confront other males to defend their territories and court females. The male repertoire includes displaying bright yellow eye combs, inflating red air sacs, flutter-jumping, cackling and foot-stamping.

Lesser prairie chickens live in southeastern Colorado; the southwestern quarter of Kansas; and in patchy areas in the panhandle and northwestern counties of Oklahoma. The species also occurs in east-central New Mexico and in small areas in the northeastern and southwestern corners of the Texas Panhandle. Kansas has the largest population of lesser prairie chickens, where the species relies heavily on habitat on private lands enrolled in the conservation reserve program.

“Lesser prairie chickens are amazing, incredibly unique birds. We should do everything we can to protect them and the prairie habitat we all share, for the sake of our children and grandchildren,” said Lininger.

The Center for Biological Diversity is a national, nonprofit conservation organization with more than 625,000 members and online activists dedicated to the protection of endangered species and wild places.